Position feedback devices, such as encoders and resolvers, for example, are used to monitor the motion of devices, such as a motor shaft, for example. Generally, the position of the shaft is monitored in terms of angular position and/or number of revolutions. Encoders typically use a code wheel attached to the shaft to modulate light as the shaft and the code wheel rotate, while resolvers use rotating windings (e.g., a rotating transformer) to determine an angular position of the shaft.
In motor control systems, an initialization routine is typically executed that enables the control software to determine a mechanical angular position of the motor rotor by reading the output of the position feedback device. Knowing the mechanical angular position of the rotor, the software can generate proper electrical commutation to most efficiently drive the motor rotor in a forward direction.
For electrical motor designs wherein the mechanical turn for a full electrical cycle is not same as that of a full position feedback turn (e.g., less than one full resolver turn, also referred to as one electrical cycle of the resolver, or the resolver turn is not an integer multiple of the electrical turn), the position feedback device does not provide a unique translation from the position feedback device to the electrical angular position for commutation. A conventional method for implementing the initialization routine is to issue electrical commands to spin the motor rotor in open loop mode to provide a desired rotational angle of the position feedback device. However, for various reasons the motor initialization may fail to establish the correct mechanical angular position and hence the electrical angle for commutation. When this happens, the motor may run inefficiently, stall, or, depending on the initial rotor position, could run backward. This initialization is needed for any motor design that does not provide a one-to-one mapping from the position feedback device reading to the electrical position determination for commutation.
For example, if a 6-pole motor is coupled to a position feedback device embodied as a resolver that has two full electrical cycles for each mechanical revolution, reading the resolver output alone does not provided certainty with regard to which mechanical half turn the resolver is referencing. The resolver indicated position and the actual mechanical position either is correct or off by 180 degrees. When the position is correct, the motor will run in the forward direction. When the position is off by 180 degrees, the motor may run backwards, depending on the initial starting position.